Brushless D.C. motors essentially comprise a rotor in the form of a multipolar permanent magnet. The rotor is rotated by electronic commutation of the current in the stator coils. For this control of the current in the stator coils it is necessary to determine the position of the rotor as precisely as possible: the more precisely the position of the rotor is known, the more precisely the total field of the stator coils may be adjusted for a smooth operation resp. the position of the rotor may be regulated e.g. if an actuator is concerned.
A known measuring method of the rotor position is to arrange Hall elements in the area of the rotor, i.e. usually near its circumference, an electric parameter of which varies in function of a magnetic field, e.g. the Hall voltage or the resistance. Unless it is provided with the magnetization for the propulsion of the rotor anyway, the area of the rotor which travels past the Hall elements receives a particular magnetization whose division and position exactly corresponds to the magnetization which serves for the propulsion.
The disadvantage of the known embodiment is that the Hall elements must be adjusted very precisely in order to obtain an optimum commutation of the stator coil current. Since the Hall elements are discrete components, each Hall element generally has to be mounted and adjusted individually. Altogether, this results in a relatively complicated construction, a laborious adjustment and high requirements with respect to the positioning precision.
According to FR-A-2 155 303, the axle e.g. of a motor is provided with a magnet assembly, and a quantitative detection of the rotation of the axle in fractions of turns is effected by means of a magnet sensor which scans the magnet assembly. In one embodiment, the magnet assembly essentially consists of two annular magnets which are disposed at a certain distance from each other and comprise a number of axial magnetizations. The disposition of the two magnets is such that an annular zone with an axial magnetic field of changing polarity is formed in the gap between the magnets while the field lines are almost parallel at the center of the gap. The multiple sensor disposed in this gap responds when a respective threshold of every sensor is exceeded or not attained, whence the position of the magnet assembly is deduced. The construction of the magnet assembly results in relatively high field intensities and sharp transitions between the different magnetization zones. The number of sensors is chosen such that the series of sensors is shorter than a complete magnetization zone.
This disposition requires an additional, special measuring magnet, and on account of the construction size, the overall conception requires magnetic field sensors in the form of discrete components. An integration of the Hall elements on a chip is not mentioned either.
EP-A-0 590 222 describes a linear position detector which comprises a number of Hall elements which are integrated in a semiconductor chip. The two respective adjacent Hall elements are determined between which the magnetic induction generated e.g. by a magnet which is displaceably arranged above the sensor passes through zero. The resolution of this detector corresponds to the distance between two Hall elements. If used for the detection of an arcuate movement, at least the problem of the tangential positioning error remains unsolved, and the increased requirements for a continuous and ungradated detection of the rotational position of the rotor of a D.C. motor for the generation of a continuous stator coil current are not mentioned.